Add documentation for data-driven styling

[boundsj]

Support for data-driven styling for iOS and macOS is being added in #7596. After we have used the API proposed there and before the final Mapbox iOS SDK 3.5.0 beta, we will fully document the new APIs added for DDS.

This will include (at least):

• MGLStyleFunctionOption string enum
  • MGLStyleFunctionOptionInterpolationBase
  • MGLStyleFunctionOptionDefaultValue
• MGLInterpolationMode enum
  • MGLInterpolationModeExponential
  • MGLInterpolationModeInterval
  • MGLInterpolationModeCategorical
  • MGLInterpolationModeIdentity
• Update class documentation for MGLStyleValue (as a class cluster it represents something different now)
• +[MGLStyleValue valueWithInterpolationMode:cameraStops:options:]
• +[MGLStyleValue valueWithInterpolationMode:sourceStops:attributeName:options:]
• +[MGLStyleValue valueWithInterpolationMode:compositeStops:attributeName:options:]
• MGLCameraStyleFunction (and all methods and properties)
• MGLSourceStyleFunction (and all methods and properties)
• MGLCompositeStyleFunction (and all methods and properties)
• Update generated code examples in MGLDocumentationExampleTests.swift to use camera functions and maybe add examples for source and composite functions
• Update guides/For Style Authors.md with DDS specific terminology differences
• Update guides/Runtime Styling.md with information about DDS (or add a DDS specific guide)

cc @friedbunny @jmkiley @1ec5 @incanus @ericrwolfe @pveugen

[1ec5]

We should fill out the documentation before the first beta, because that's when we tell developers to start coding against new features in nontrivial projects. We can ship an alpha without the documentation if necessary to meet time commitments, however.

[1ec5]

• Update the iOS and macOS changelogs to discuss all that’s changed

[boundsj]

#7931, #7943, and #7960 added API documentation that should be suitable for the beta release. We still need a PR to update the guides but I don't consider that a requirement for the beta.

Before I create the beta, I will wait for:

• Update changelogs for DDS #7993 that adds changelog updates for iOS and macOS.
• Add function and interpolation mode to style property docs #7958 that adds notes about the possible function and interpolation modes combinations that are valid for each property

[1ec5]

#8025 will update the MGLFeature documentation to discuss more ways a feature’s attributes can be used, including in source functions.

[boundsj]

I'm removing release blocker and closing this issue. The work to update and add guides continues in #8354 and it is possible that that PR may also be sufficient for #8192.

cc @jmkiley